The application relates to a semiconductor device made of silicon with regionally reduced band gap and a process for the production of same.
A semiconductor device made of silicon with a regionally reduced band gap is known from PCT/US2005/036036 which discloses a transistor with MOS-gate including a body region with a source region and a drain region which forms a pn-junction with the body region, the source having a lower energy gap than the body region. The lower energy gap in the source region is achieved due to the fact that, although the source zone remains highly n-doped, it consists of a binary compound semiconductor containing silicon and germanium and thus provides a band gap for the source zone which is smaller than the silicon band gap but larger than the germanium band gap. This weakens a parasitic bipolar transistor and thus improves the avalanche behaviour of the semiconductor device.
The production of a semiconductor device with a band gap in the source zone presents significant difficulties to be resolved in that following production of the source zone further high temperature processes are still required to produce a MOS gate transistor of this type. However, these high temperature processes create a risk that germanium will diffuse out of the silicon lattice in processes above 600° C., thereby rendering it impossible to maintain the desired reduced band gap in the source zone.
Other solutions for reducing the flow voltage drop of the body diode occurring at the pn-junction between the body zone and the drain zone which are known from the prior art are based on the parallel connection of an additional diode with a lower flow voltage parallel to the body diode. These parallel-connected diodes with lower flow voltage can either be connected externally in the form of germanium diodes or integrated in the semiconductor chip in the form of Schottky diodes. However, integrated Schottky diodes reduce the semiconductor area available for the MOSFET and the use of Schottky transitions results in a higher area-specific closing resistance.
In addition to the aforementioned germanium diodes of lesser band gap which can be connected externally in parallel to the body/drain pn-junction, it is also possible to integrate germanium diodes of this type with a typical flow voltage of 0.2 to 0.3 V on a silicon chip. However, germanium diodes integrated in this manner would have a relatively high leakage current when blocked, and due to the small band gap operating temperatures would have to be limited to below 90° C., temperatures unacceptable for power semiconductor devices. In addition, the leakage currents of pure germanium diodes increase exponentially at temperatures above 50° C.
Integrated diodes made of an SiGe binary compound semiconductor also present certain disadvantages relating to the thicknesses of the various layers needed for a silicon-germanium diode which necessarily require a buffer layer if SiGe is to be grown on a monocrystalline silicon crystal region, for example, in order to avoid crystal defects and to switch the lattice constants of the monocrystalline silicon to the lattice constants of the binary compound semiconductor SiGe. If this buffer layer is accommodated in the body region, for example, the flow voltage of the body diode increases. At the same time, the thickness of the useful SiGe layer is limited and too low for the layer thicknesses required for power semiconductor devices with an integrated SiGe diode made of a binary compound semiconductor material.
For these and other reasons, there is a need for the present invention.